Velocity and Acceleration Freely Tunable Straight-Line Propagation
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www.nature.com/scientificreports OPEN Velocity and acceleration freely tunable straight‑line propagation light bullet Zhaoyang Li* & Junji Kawanaka Three-dimensional (3-D) light solitons in space–time, referred to as light bullets, have many novel properties and wide applications. Here we theoretically show how the combination of difraction-free beam and ultrashort pulse spatiotemporal‑coupling enables the creation of a straight‑line propagation light bullet with freely tunable velocity and acceleration. This light bullet could propagate with a constant superluminal or subluminal velocity, and it could also counter-propagate with a very fast superluminal velocity (e.g., − 35.6c). Apart from uniform motion, an acceleration or deceleration straight‑line propagation light bullet with a tunable instantaneous acceleration could also be produced. The high controllability of the velocity and the acceleration of a straight-line propagation light bullet would enable very specifc applications, such as velocity and/or acceleration matched micromanipulation, microscopy, particle acceleration, radiation generation, and so on. Te combination of difraction-free beam and dispersion-free pulse permits the chance to produce 3-D self- similar spatiotemporal optical wave packets (light bullets), which could propagate over long distances and, importantly, maintain invariant intensity profles in space and time1–4. In nonlinear optics, the balance between nonlinearity and dispersion/difraction could easily result in temporal/spatial solitons 5–13, however some appli- cations require to generate light bullets in linear/free space. In linear optics, Bessel beam is one of the most important difraction-free beams whose central core is propagation invariant 14,15, and up to now many novel methods have already been proposed16–21, such as circular slit with lens16, axicon17, spatial light modulator18, and so on. Airy beam is another interesting difraction-free beam, and, compared with Bessel beam, its main intensity maxima and lobes could tend to accelerate during propagation along a parabolic trajectory 22–29. For a difraction-free beam, when the monochromatic wave is replaced by a dispersion-free pulse, a light bullet might be produced. Te simplest form is the Gauss-Bessel/Airy light bullet in free space (non-dispersion environment) or the Bessel/Airy-Bessel/Airy light bullet in materials (dispersion environment) 1,22,30,31. To control the velocity and even the acceleration of a light bullet is an interesting but challenging work. Previously, during a short propa- gation distance, the group velocity given by υg = c/ng could be controlled by crafing the wavelength-dependent refractive index32, however some very special material or photonic systems are necessary33–41, such as ultra-cold atoms33, hot atomic vapors35, stimulated Brillouin scattering36, active gain resonances37, tunneling junctions38, metamaterials39, photonic crystals40, and so on. Te problem is that, in transparent materials at wavelengths far from resonance or even in free space, the controllability of ng and accordingly the group velocity υg would be very limited. Light bullet with an airy beam could self-accelerate during propagation in free space, however which travels along a bended (parabolic) propagation trajectory 22–24. Recently, several spatiotemporal coupling methods have been proposed to control the group velocity of optical wave packets in free space42–46. For example by controlling temporal and spatial dispersion (temporal chirp and chromatic aberration)42,44, a fying focus with a tunable group velocity could be achieved, however the propagation distance is limited within the focusing region. In this article, we combined the frst-order spatiotemporal coupling (pulse-front pre-deformation) with the difraction-free (Gauss-Bessel) pulsed beam and theoretically produced a straight-line propagation light bullet in free space, whose velocity and acceleration could be freely controlled, including all cases of superlumi- nal, subluminal, acceleration, deceleration and backwards-traveling group velocities. We have mainly discussed the case of a Gauss-Bessel pulsed beam in free space, and the method is also suitable to other types of pulsed beams, for example the Airy-Bessel pulsed beam in free space or materials. Tis velocity and acceleration tunable straight-line propagation light bullet has lots of specifc applications from basic sciences to industry applications. Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan. * email: [email protected] SCIENTIFIC REPORTS | (2020) 10:11481 | https://doi.org/10.1038/s41598-020-68478-1 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1. Gauss-Bessel light bullet with a conical-pulse-front pre-deformation. (a) Te input pulsed beam possesses a plane phase-front and a conical pulse-front, and the axicon generates a Gauss-Bessel light bullet (in free space) in the interference region. (b) Superluminal light bullet in the case of CC-CPF (α = 0.5° and β = 6.6°). Te longitudinal gap Δz (between the light bullet and the intersection of phase-fronts illustrated by white lines) decreases during propagation. (c) Subluminal light bullet in the case of CV-CPF (α = 0.5° and β = − 6.6°). Te longitudinal gap Δz increases during propagation. (d) Variation of the longitudinal gap Δz with time t. Te travelling velocity of the intersection of phase-fronts is 1.00004c, and that of the light bullet with CC-CPF of (b) and CV-CPF of (c) is 1.001c and 0.999c, respectively. Superluminal and subluminal light bullets. An axicon can transform a plane wave into a conical wave and generate a Bessel beam in the overlap region due to interference 17. In vacuum, the propagation velocity (group velocity) of this Bessel beam is υb = c/cosα, where c is the light speed in vacuum and α is the (half) conical angle (relevant to both the refractive index and the wedge angle of the axicon)47. It can be found that this veloc- ity is faster than the light speed in vacuum c, which also increases with increasing the conical angle α. Figure 1a shows the key idea of the method proposed in this article that the input pulsed beam possesses a distortion-free (plane) phase-front but a pre-deformed (conical) pulse-front, which is quite diferent from the previous case with both distortion-free (plane) phase- and pulse-fronts. It is necessary to introduce that the phase-front is the surface perpendicular to the propagation direction while the pulse-front is the surface coinciding with the peak of a pulse, which are respectively determined by the phase- and group-velocities48. Te result of the method is that the velocity of the generated light bullet could be freely controlled. As an example we begin the simulation with an initial input pulsed beam with a 30 fs (FWHM) pulse in time, a 2 mm (1/e2) beam in space and a concave conical-pulse-front (CC-CPF) in space–time, and the generation and the propagation of the light bullet in the 2-D propagation section of the x–z plane is shown in Fig. 1b. Te axicon spatially divides the input pulsed beam into two Gauss-Gauss ones and changes their travelling directions (green arrows) with symmetrical angles of α = ± 0.5°. Te phase-fronts of two pulsed beams are illustrated by white lines, and the pulse-fronts are shown by red distributions. It can be found that in the overlap region a light bullet (in free space) is generated due to inter- ference, which in time is a Gauss pulse and in space is a Bessel beam. Because the pulse-front is pre-deformed to deviate from the phase-front with a tilt angle of β = 6.6°, the generated light bullet is not located at the inter- section of the phase-fronts anymore, and we can say the light bullet is temporally or spatially delayed along the longitudinal axis. Here, we make two defnitions: in space, the propagation (longitudinal) axis of the Bessel beam is the z–axis and its geometrical center in the overlap region (formed by two pulsed beams afer the axicon) is the SCIENTIFIC REPORTS | (2020) 10:11481 | https://doi.org/10.1038/s41598-020-68478-1 2 Vol:.(1234567890) www.nature.com/scientificreports/ Figure 2. Controllability of velocity and backwards travelling light bullet. (a) Velocity of the light bullet as a function of the pulse-front tilt angle β for diferent conical angles α (α equals 0.5°, 1° & 5° in upper and 80°, 85° & 88° in lower). Te velocity is normalized by c. (b) Backwards travelling light bullet with α = 85° and β = 6.6° (spot marked in (a) lower), and the velocity is − 35.6c. Due to a large conical angle α, the light bullet is very small and most energies are transferred to the rings around the light bullet. White lines and arrows in (b) illustrate phase-fronts and propagation directions. origin of z = 0; and in time, the moment when the intersection of two phase-fronts (the original location of the light bullet if without any pulse-front pre-deformation) arriving at z = 0 is the zero time of t = 0. Figure 1b gives the detailed distributions of the pulse-fronts, the phase-fronts and the light bullet at diferent times of t = − 120, − 60, 0, 60 and 120 ps during propagation. According to the previous result41, the intersection of two phase- fronts travels at a velocity of 1.00004c governed by υb = c/cosα. However, due to the pulse-front pre-deformation, the light bullet is behind the intersection of phase-fronts and the longitudinal gap Δz (distance between the light bullet and the intersection of phase-fronts) decreases with time during propagation. Tis phenomenon indicates the travelling velocity of the light bullet is faster than that of the intersection of phase-fronts, i.e., a superluminal light bullet is produced.